1. Field of the Invention
This invention relates to sodar methods and apparatus for atmospheric sounding using encoded pulses in the transmit-then-listen mode. The techniques of the invention have particular application to so-called ‘monostatic’ sodar configurations in which the receiver(s) is located ‘up-range’ near the transmitter and backscattered echoes are detected by the receiver. However, the methods of the invention are also applicable to so-called ‘bistatic’ sodar configurations where the receiver(s) is located down-range and forward-scattered echoes are detected.
2. Description of Related Art
Transmit-then-listen sodar sounding techniques, in which the transmission of a short high-intensity acoustic pulse is followed by a longer listening period in which echoes from the pulse are detected, are standard in the prior art and have been known for nearly 100 years. The basic problem with such techniques is that the pulse must be very short—preferably a few milliseconds—so that the transmit pulse ends before the echoes from short-range anomalies are returned and detected, otherwise the energy of the transmitted pulse will swamp the receiver. This is a particularly important consideration in monostatic systems where the receiver is located close to the transmitter. Two problems associated with the use of short pulses: they must have very high energy to achieve an acceptable signal-to-noise ratio [s/n], and they are difficult to encode in any effective way so that pulse-compression techniques are difficult to apply.
In our prior international applications PCT/AU01/00247, PCT/AU02/01129 and PCT/AU04/00242 we disclosed sodar techniques using long pulse-compression-encoded transmitted pulses—called ‘chirps’ for short—that allowed ‘listening while sending’, so long as the received echoes were digitally processed using matched filters tailored to the encoding used in the chirps. Matched filtering is a well know technique practiced in radar and may be effected in the frequency domain (sometimes called the Fourier domain) or in the time domain by correlation methods. In the work associated with our prior patent applications, Chirp lengths of more than 40 seconds were demonstrated, the length being dependent upon the processing power devoted to the DSP (digital signal processing) techniques that implement the matched filtering. Very high s/n was achieved relative to the conventional transmit-then-listen methods as the total energy of a long chirped pulse can often be 1000 times that of a very short high-intensity pulse used in send-then-listen techniques.
However, we believe that there are situations where there may be advantage in using a hybrid system; that is, one which operates on a send-then-listen basis but has at least some of the s/n advantages of a chirped, long-pulse listen-while-sending system of the type disclosed in our above-mentioned prior patents.